1. Field of the Invention
The present invention relates to a nonmagnetic black toner for reversal development used for the development of a latent image formed in electrophotography, electrostatic recording method, electrostatic printing method or the like.
2. Discussion of the Related Art
Conventionally, carbon blacks have been used as a black colorant for a toner. However, the carbon blacks have some defects such that the volume specific resistance is low, so that triboelectric charges required for development cannot be maintained, whereby a sufficient degree of blackness cannot be obtained. In addition, there are also pointed out some problems in safety hygiene. Therefore, various composite oxides have been proposed as black colorants used in place of carbon black (Japanese Patent Laid-Open No. 2000-10344 (U.S. Pat. No. 6,130,017) and Japanese Patent Laid-Open No. Hei 9-25126.
On the other hand, recently, similar to the widespread trends in plain paper copy machines (PPC), there has been a remarkable progress in laser beam printers (LBP). In the case of the PPC, the development is carried out by forming an electrostatic latent image carrying electric charges on a photoconductor, and changing its surface potential by the intensity of the light source, thereby changing the image tone (charged area development). By contrast, in the case of LBP, since a latent image not having electric charges is formed by two-step of on-and-off, the area coverage modulation by the number of halftones is carried out (discharged area development, i.e. reversal development). Therefore, in the reversal development, the transferability of fine halftones affects the clearness, so that an improvement in image transferability is especially desired.
Conventionally, proposals for improving the image transferability, including a toner in which its wettability is adjusted by an amount of a wax or the like (Japanese Patent Laid-Open No. Hei 7-104503), a toner in which a silica having a large size is added (Japanese Patent Laid-Open No. Hei 7-271087), and the like, have been made. However, these toners have some defects such that filming of the toner is likely to take place in the former toner, and that the silica is embedded in the toner, so that its durability tends to be lowered in the latter toner.
An object of the present invention is to provide a nonmagnetic black toner for reversal development, comprising a black colorant useful for reversal development, namely a nonmagnetic black toner for reversal development for performing area coverage modulation by halftone, which has a sufficient high degree of blackness, a high volume-specific resistance, and excellent image transferability.
These and other objects of the present invention will be apparent from the following description.
According to the present invention, there is provided a nonmagnetic black toner for reversal development comprising:
a resin binder; and
a black colorant comprising a composite oxide of two or more metals, the composite oxide having an oil absorption per unit area of 0.07 ml/m2 or less.
One of the greatest features of the toner of the present invention resides in that the toner comprises a black colorant comprising a composite oxide of two or more metals, the composite oxide having a specified oil absorption. By adjusting the oil absorption of the composite oxide, the affinity of the composite oxide with the resin binder is adjusted, whereby the dispersibility of the composite oxide can be increased. By the improvement in the dispersibility of the composite oxide, the toner can be made into a smaller size, and the transferability of the toner is improved together with the uniform chargeability and the stability with the passage of time. Therefore, the composite oxide has an oil absorption per unit area of 0.07 ml/m2 or less, preferably from 0.0001 to 0.05 ml/m2, more preferably from 0.001 to 0.02 ml/m2. In the present invention, the above-mentioned oil absorption (ml/m2) is calculated by the following equation using the oil absorption (ml/100 g) as determined by the method according to JIS K5101 and the specific surface area (m2/100 g):                               Oil          ⁢                      xe2x80x83                    ⁢          Absorption          ⁢                      xe2x80x83                    ⁢          Per                                              Unit          ⁢                      xe2x80x83                    ⁢          Area          ⁢                      xe2x80x83                    ⁢                      (                          ml              ⁢                              /                            ⁢                              m                2                                      )                                =            Oil      ⁢              xe2x80x83            ⁢      Absorption      ⁢              xe2x80x83            ⁢              (                  ml          ⁢                      /                    ⁢          100          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          g                )                    Specific      ⁢              xe2x80x83            ⁢      Surface      ⁢              xe2x80x83            ⁢              Area        ⁢                              xe2x80x83                    ⁢                      xe2x80x83                          (                              m            2                    ⁢                      /                    ⁢          100          ⁢                      xe2x80x83                    ⁢          g                )            
The oil absorption of the composite oxide, which may be dependent on its composition, is especially greatly dependent on its particle size. When the specific surface area becomes larger by making the particle size smaller, the oil-absorption also becomes larger. On the other hand, when the specific surface area becomes smaller by making the particle size larger, the oil-absorption also becomes smaller. In addition, the oil absorption can be increased by utilizing capillary phenomenon by the secondary aggregation.
The composite oxide has an average particle size of preferably from 5 nm to 1 xcexcm, more preferably from 5 to 500 nm, especially preferably from 5 to 200 nm, from the viewpoints of the oil absorption and the covering strength.
In the present invention, the composite oxide is constituted by at least 2 metals, preferably at least 3 metals, from the viewpoint of the degree of blackness of the toner. Especially, it is preferable that at least one, preferably at least two, more preferably at least three of the metals of the composite oxide belongs to Group 2 or 13 of the Third Period of the Periodic Table, or to Groups 3 to 11 of the Fourth Period of the Periodic Table. Magnesium (Mg) and aluminum (Al) belong to Group 2 or 13 of the Third Period of the Periodic Table, and scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni) and copper (Cu) belong to Groups 3 to 11 of the Fourth Period of the Periodic Table. Among them, Mg, Al, Ti, Mn, Fe and Cu are preferable, and Mg, Al, Mn, Fe and Cu are especially preferable. The compositional ratio of the metals in the composite oxide is not particularly limited.
The content of the composite oxide is preferably from 4 to 30% by weight, more preferably from 4 to 20% by weight, especially preferably from 7 to 15% by weight, of the toner, from the viewpoints of the degree of blackness and the specific gravity of the toner.
The process for preparing a composite oxide includes a process comprising depositing other oxide on a surface of the main oxide used as a core particle (Japanese Patent Laid-Open No. 2000-10344 (U.S. Pat. No. 6,130,017)), a process of making a composite oxide comprising sintering several oxides (Japanese Patent Laid-Open No. Hei 9-25126), and the like, without being particularly limited thereto.
The preferable commercially available composite oxide in the present invention includes xe2x80x9cDye Pyroxide Black No. 1,xe2x80x9d xe2x80x9cDye Pyroxide Black No. 2xe2x80x9d (hereinabove commercially available from DAINICHISEIKA COLOR and CHEMICALS MFG. CO., LTD.), xe2x80x9cHSB-603Rx,xe2x80x9d xe2x80x9cHSB-605xe2x80x9d (hereinabove commercially available from Toda Kogyo Corp.), xe2x80x9cETB-100xe2x80x9d (commercially available from Titan Kogyo K.K.), MC Series (commercially available from MITSUI MINING and SMELTING CO., LTD.), and the like.
The toner of the present invention may contain a known colorant other than the above-mentioned composite oxide as a colorant, but it is preferable that carbon black is not contained.
The resin binder in the present invention includes polyesters, hybrid resins which are defined below, styrene-acrylic resins, epoxy resins, polycarbonates, polyurethanes, and the like, without being particularly limited thereto. Among them, from the viewpoints of the dispersibility and the transferability of the colorant, the polyester and the hybrid resin are preferable, and the polyester is more preferable. The content of the polyester or the hybrid resin is preferably from 50 to 100% by weight, more preferably from 80 to 100% by weight, especially preferably 100% by weight, of the resin binder.
The term xe2x80x9chybrid resinxe2x80x9d as referred to herein is a resin in which a condensation polymerization resin component, such as a polyester, is partially chemically bonded with an addition polymerization resin component such as a vinyl resin. The hybrid resin may be obtained by using two or more resins as raw materials, or it may be obtained by using one resin and raw material monomers of the other resin. Further, the hybrid resin may be obtained from a mixture of raw material monomers of two or more resins. In order to efficiently obtain a hybrid resin, those obtained from a mixture of raw material monomers of two or more resins are preferable.
The raw material monomer for the polyester includes dihydric or higher polyhydric alcohols and dicarboxylic or higher polycarboxylic acid compounds.
The dihydric alcohol includes, for instance, alkylene oxide adducts of bisphenol A such as polyoxypropylene(2,2)-2,2-bis(4-hydroxyphenyl)propane and polyoxyethylene(2,2)-2,2-bis(4-hydroxyphenyl)propane, ethylene glycol, 1,2-propylene glycol, 1,4-butanediol, neopentyl glycol, polyethylene glycol, polypropylene glycol, bisphenol A, hydrogenated bisphenol A, and the like.
The trihydric or higher polyhydric alcohol includes, for instance, sorbitol, pentaerythritol, glycerol, trimethylolpropane, and the like.
In addition, the dicarboxylic acid compound includes, for instance, dicarboxylic acids such as maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid, adipic acid, and succinic acid; a substituted succinic acid of which substituent is an alkyl group having 1 to 20 carbon atoms or an alkenyl group having 2 to 20 carbon atoms, such as tetrapropenylsuccinic acid, n-dodecenylsuccinic acid, isododecenylsuccinic acid, n-dodecylsuccinic acid, isooctenylsuccinic acid and isooctylsuccinic acid; acid anhydrides thereof or lower alkyl(1 to 3 carbon atoms) esters thereof; and the like.
The tricarboxylic or higher polycarboxylic acid compound includes, for instance, 1,2,4-benzenetricarboxylic acid (trimellitic acid), 2,5,7-naphthalenetricarboxylic acid, pyromellitic acid, acid anhydrides, lower alkyl(1 to 3 carbon atoms) esters thereof, and the like.
The polyester can be prepared by, for instance, polycondensation of an alcoholic component, a carboxylic acid compound and the like at a temperature of 180xc2x0 to 250xc2x0 C. in an inert gas atmosphere in the presence of an esterification catalyst as desired.
It is desired that the polyester has an acid value of from 0.5 to 60 mg KOH/g, from the viewpoint of the dispersibility and the transferability of the colorant, and that the polyester has a hydroxyl value of from 1 to 60 mg KOH/g.
In addition, the polyester has a softening point of 80xc2x0 to 165xc2x0 C., and a glass transition point of 50xc2x0 to 85xc2x0 C.
The toner of the present invention may appropriately contain, in addition to the resin binder and the colorant, an additive such as a charge control agent, a fluidity improver, a releasing agent, an electric conductivity modifier, an extender, a reinforcing filler such as a fibrous substance, an antioxidant, an anti-aging agent, and a cleanability improver.
The toner of the present invention can be prepared by any of conventionally known methods such as kneading and pulverization method, polymerization method, emulsion and phase inversion method. Concretely, in a case of a pulverized toner prepared by kneading and pulverization method, for instance, the method comprises homogeneously mixing a resin binder, a colorant, and the like in a mixer such as a Henschel mixer or a ball-mill, thereafter melt-kneading with a closed kneader or a single-screw or twin-screw extruder, cooling, pulverizing and classifying the product. The volume-average particle size of the toner is preferably from 3 to 15 xcexcm. Further, a fluidity improver such as hydrophobic silica or the like may be added to the surface of the toner as an external additive as occasion demands.
The nonmagnetic black toner of the present invention can be made into a small size by the improvement in the dispersibility of the composite oxide, and the transferability of the toner is improved together with the uniform chargeability and the stability with the passage of time, so that the transferring of fine halftones can be facilitated, thereby making it highly useful as a toner for reversal development. Since the triboelectric charges can be stably maintained, the toner can be also preferably used in the nonmagnetic monocomponent development. In the present invention, the term xe2x80x9cnonmagnetic tonerxe2x80x9d refers to a paramagnetic material, a diamagnetic material, or a ferromagnetic material having a saturation magnetization of 10 Am2/kg or less, preferably 2.5 Am2/kg or less.
Further, the nonmagnetic black toner for reversal development of the present invention is similar to the resistance of colorants such as yellow, cyan and magenta, the nonmagnetic black toner can be suitably used in the formation of full-color fixed images.
Furthermore, the present invention provides a process for development of a toner, comprising applying the nonmagnetic black toner of the present invention to a development device for reversal development. In this process, it is preferable that the development device is a device for nonmagnetic monocomponent development, or a device for full-color development.